System and method for controlling the temperature of a catheter-mounted heater

ABSTRACT

A system for keeping the surface temperature of an electric resistance-type heater element in a thermodilution catheter within safe physiological limits includes, in the preferred embodiment, a heater element core temperature monitor, a monitor for monitoring the power that is supplied to the heater element, and a surface temperature calculator for calculating the surface temperature of the heater element based on the core temperature, supplied power, and information representing the characteristics of the particular catheter under anticipated clinical conditions. A second aspect of the invention involves a system for determining the supply of power to the heater element based on the core temperature of the heater element. A third aspect of the invention involves a system readiness test for determining, in vivo, that the thermodilution catheter system is properly calibrated before the system is operational. Methods of operation for each of the above-referenced aspects of the invention are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/334,443,filed Nov. 4, 1994, now U.S. Pat. No. 5,553,622 which is a continuationof application Ser. No. 07/833,013, filed Feb. 10, 1992, now abandoned,which is a continuation-in-part of Ser. No. 07/647,578, entitled "AThermodilution Catheter Having a Safe, Flexible Heating Element," filedon Jan. 29, 1991, now abandoned. The disclosure of that application ishereby incorporated by reference as if set forth fully herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to thermodilution catheters of the typethat have an electric resistance-type heating element for applying heatto a patient's blood for purposes of measuring a physiologicalcondition, such as volumetric blood flow. More specifically, theinvention relates to systems and methods for maintaining the surfacetemperature of such a heating element at a level which will not beharmful to a patient.

2. Description of the Prior Art

Catheters have long been used for applying therapeutic or diagnosticpreparations directly into the blood stream of animals or humans.Catheters are also commonly used to measure such parameters as cardiacoutput, blood pressure, blood volume, blood components and the like.

Numerous techniques have been disclosed in the prior art for measuringblood flow using catheters. One such technique, termed indicatordilution, relies on the introduction of a marker into the bloodstream,the theory being that the marker will dissipate at a rate which is afunction of blood flow as measured in units of volume per unit of time.

The present inventors believe, clinically, that heat is the preferredmarker for such an indicator dilution system. Unlike other indicators,heat is conserved in the immediate vascular system, but is largelydissipated in the periphery in one circulation time so as to eliminaterecirculation and accumulation problems. Cold (negative heat) is anindicator which can also be used very effectively in a clinical setting.Large amounts of cold may be used, for cold has relatively nodeleterious effects on blood and surrounding tissues. However, adisadvantage of cold as an indicator is that it must be supplied in achilled fluid carrier such as saline, because cold producing transducersare not commercially available. Cold-based indicator systems aredisclosed in U.S. Pat. No. 4,819,655 to Webler and in U.S. Pat. No.4,941,475 to Williams. Both of those systems have significant clinicallimitations in that the circulating fluid must be cooled to near icetemperature prior to input into the catheter and temperature equilibriummust be established, which takes a significant amount of time. Inaddition, the enlarged catheter segment which is necessary forcontaining the cooling elements may restrict blood flow.

A disadvantage of heat as an indicator is that even small increases inheat transducer temperature can have a deleterious effect on blood andlocal tissue. In fact, it can be inferred from the teachings of Ham etal. in "Studies in Destruction of Red Blood Cells, Chapter IV. ThermalInjury", Blood, Vol. 3, pp. 373-403 (1948), by Ponder in "Shape andTransformations of Heated Human Red Cells", J. Exp. Biol., Vol. 26, pp.35-45 (1950) and by Williamson et al. in "The Influence of Temperatureon Red Cell Deformability", Blood, Vol. 46, pp. 611-624 (1975), that amaximum safe filament surface temperature is probably about 48° C.

A heater element in a catheter must satisfy several requirements if itis to be used clinically. Most importantly, the heat transducer orfilament must be electrically safe. It also must only minimally increasethe catheter cross-sectional area or diameter of the catheter and mustbe made of materials which are non-toxic and can be sterilized. Such aheater element must also be flexible so as not to increase the stiffnessof the catheter body.

In prior art heat-type thermodilution catheters, either the heaterelement temperature is not monitored, or the temperature is measuredwith a second thermometer. Use of a second thermistor significantly addsto the cost of the catheter and provides a temperature measurement, butonly at a single point. Accordingly, the measured temperature might notbe representative of the surface temperature as a whole. Not monitoringheat or temperature does not allow for detection of undesirable events(e.g. low flow condition).

Gibbs, in an article entitled "A Thermoelectric Blood Flow Recorder inthe Form of a Needle", Proc. Soc. Exp. Biol. & Med., Vol. 31, 1933,Pages 141-146, has suggested using the principle upon which a hot-wireanemometer operates to measure blood velocity. However, as noted byGibbs in that article, such a technique has been limited to peripheralvessels and cannot give absolute blood volumetric flow rates, onlyvelocity.

It is clear that there exists a long and unfilled need in the prior artfor an improved system for maintaining the surface temperature of athermodilution catheter heater element within safe physiological limits.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a system formaintaining the surface temperature of a thermodilution catheter heaterelement within safe physiological limits.

It is further an object of the invention to provide such a system, whichdoes not necessitate a secondary temperature measuring transducer formonitoring the surface temperature of the heater element.

It is further an object of the invention to provide a system formaintaining the surface temperature of a thermodilution catheter heaterelement which adjusts the supply of power to the catheter heater elementin response to the core temperature of the heater element.

It is yet further an object of the invention to provide a system formaintaining the surface temperature of a thermodilution catheter heaterelement which is capable of testing the accuracy of its calibrationprior to operation.

To achieve the above-referenced and other objects of the invention notspecifically set forth, a system according to a first aspect of theinvention for keeping the surface temperature of an electricresistance-type heater element in a thermodilution catheter within safephysiological limits includes a core temperature monitor for monitoringa core temperature of the electric resistance-type heater element; apower monitor for monitoring the amount of electric power that issupplied to the heater element; surface temperature calculatingstructure, in communication with the core temperature monitor and thepower monitor, for calculating the surface temperature of the heaterelement; and detection structure, in communication with the surfacetemperature calculating structure, for determining whether a potentialphysiologically harmful temperature condition exists.

According to a second aspect of the invention, a system for keeping thetemperature of an electric resistance-type heater element in athermodilution catheter within safe physiological limits includes a coretemperature monitor for monitoring a core temperature of the electricresistance-type heater element; a power monitor for monitoring theamount of electric power that is supplied to the heater element; a powersource for supplying power to the heater element; and control structurein communication with the core temperature monitor and with the powermonitor for controlling the amount of power that is supplied to theheater element by the power source, whereby the temperature of theheater element is kept within safe physiological limits.

According to a third aspect of the invention, a heater resistanceverification system for verifying, in vivo, the calibration of athermodilution catheter system of the type which utilizes an electricresistance-type heater element includes structure for measuring, invivo, the temperature of blood which is in contact with the catheter; apower supply for supplying electric power to the heater element; a powermonitor for monitoring the amount of electric power that is supplied tothe heater element by the power supply; a resistance monitor formonitoring the electrical resistance of the heater element; and controlstructure in communication with the temperature measuring means, thepower supply, the power monitor and the resistance monitor for (a)empirically determining the relationship between supplied power andheater element resistance under the in vivo conditions; (b) using theempirically determined relationship to estimate what heater elementresistance would be at a reference temperature; (c) comparing theestimated heater element resistance at the reference temperature withthe known heater element resistance at the reference temperature; and(d) determining whether the difference between the estimated heaterelement resistance and the known heater element resistance exceeds apredetermined maximum.

A method according to a fourth aspect of the invention for keeping thesurface temperature of an electric resistance-type heater element in athermodilution catheter within safe physiological limits includes thesteps of:(a) monitoring a core temperature of the electricresistance-type heater element; (b) monitoring the amount of electricpower that is supplied to the heater element;(c) calculating the surfacetemperature of the heater element, based at least in part on coretemperature and power; and (d) determining, based at least in part onsurface temperature, whether a potential physiologically harmfultemperature condition exists.

A method according to a fifth aspect of the invention for keeping thetemperature of an electric resistance-type heater element in athermodilution catheter within safe physiological limits includes thesteps of: (a) monitoring a core temperature of the electricresistance-type heater element; (b) monitoring the amount of electricpower that is supplied to the heater element; and (c) controlling theamount of power that is supplied to the heater element based on the coretemperature of the heater element, whereby the temperature of the heaterelement is kept within safe physiological limits.

A heater resistance verification method according to a sixth aspect ofthe invention for verifying, in vivo, the calibration of athermodilution catheter system of the type which utilizes an electricresistance-type heater element includes the steps of: (a) empiricallydetermining the relationship between supplied power and heater elementresistance under the in vivo conditions; (b) using the empiricallydetermined relationship to estimate what heater element resistance wouldbe at a reference temperature; (c) comparing the estimated heaterelement resistance at the reference temperature with the known heaterelement resistance at the reference temperature; and (d) determiningwhether the difference between the estimated heater element resistanceand the known heater element resistance exceeds a predetermined maximum.

A system for measuring the surface temperature of an electricresistance-type heater element in a thermodilution catheter according toa seventh aspect of the invention includes a core temperature monitorfor monitoring a core temperature of the electric resistance-type heaterelement; a power monitor for monitoring the amount of electric powerthat is supplied to the heater element; and surface temperaturecalculating means, in communication with the core temperature monitorand the power monitor, for calculating the surface temperature of theheater element. These and various other advantages and features ofnovelty which characterize the invention are pointed out withparticularity in the claims annexed hereto and forming a part hereof.However, for a better understanding of the invention, its advantages,and the objects obtained by its use, reference should be made to thedrawings which form a further part hereof, and to the accompanyingdescriptive matter, in which there is illustrated and described apreferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 an overall perspective view illustrating the proximal end of acatheter for measuring cardiac output in accordance with the presentinvention;

FIG. 2 illustrates a cross-sectional view of the catheter of FIG. 1showing the filament lead lumen which receives the heating filamentleads and/or heating element in accordance with the invention;

FIG. 3 illustrates a detailed view of the heater connector in thecatheter of FIG. 1;

FIG. 4(a) illustrates a first embodiment of a distal end of the catheterof the invention for use in flow-directed measurement wherein theheating filament is wound about a body wall portion of the catheter andis enclosed within an outer sheath;

FIG. 4(b) illustrates a modification of the first embodiment wherein theheating filament is flush with the adjacent section of the catheter bodyso as to prevent an increase in the catheter cross-section;

FIG. 5 illustrates a second embodiment of a distal end of the catheterof the invention for use in retro grade measurement wherein the heatingfilament is wound about a body wall portion of the catheter and isenclosed within an outer sheath;

FIG. 6 illustrates a third embodiment of a distal end of the catheter ofthe invention for use in retro grade measurement wherein a "pigtail" tipis provided to prevent blood vessel rupture;

FIG. 7 illustrates an embodiment wherein the heater element and itssupporting sheath are inserted into the lumen of the catheter of FIG. 1;

FIG. 8 illustrates a calibration circuit having a ROM in accordance witha preferred embodiment of the invention;

FIG. 9 a graphical depiction of core temperature versus flow for aparticular thermodilution catheter heater element in turbulent flow testconditions;

FIG. 10 is a graphical depiction of core temperature versus flow for thesame catheter element depicted in FIG. 9 under laminar flow testconditions;

FIG. 11 a graphical depiction of surface temperature versus flow for thecatheter heater element depicted in FIGS. 9 and 10 under turbulent flowtest conditions;

FIG. 12 is a graphical depiction of surface temperature versus flow forthe catheter heater element depicted in FIGS. 9-11 under laminar flowtest conditions;

FIG. 13 is a graphical depiction of surface versus core temperature,representing the average of the turbulent and laminar data sets depictedin FIGS. 9-12;

FIG. 14 is a graphical depiction of core temperature versus power forthe two state flow dependent power control method according to apreferred embodiment of the invention;

FIG. 15 is a schematic flow diagram depicting the power control andsafety shut-off systems and methods according to a preferred embodimentof the invention;

FIG. 16 is a graphical depiction of heater element resistance versuspower at constant flows, measured with a dc power supply and testinstruments; and

FIG. 17 is a graphical depiction of heat element resistance versus powerat constant flows, as estimated by the heater resistance verificationsystem according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

1. Description of the Embodiments Shown in FIGS. 1-8

A system in accordance with preferred exemplary embodiments of theinvention will be described below in detail with reference to FIGS. 1-8.It will be appreciated by those of ordinary skill in the art that thedescription given herein with respect to those embodiments is forexemplary purposes only and is not intended in any way to limit thescope of the invention. All questions regarding the scope of theinvention may be resolved by referring to the appended claims.

A detailed description of intra vascular catheters is not given herein,for the features of different types of catheters, namely flow-directedpulmonary artery catheters, left ventricular angiography catheters, andthe like are well known to those familiar to the art. Some uniquefeatures of such catheters are described by way of example in U.S. Pat.Nos. 3,746,003; 3,634,924; 3,995,623; 4,696,304; 4,718,423; and4,721,115.

FIG. 1 illustrates a proximal end of a catheter arrangement 10 inaccordance with a first embodiment of the invention. As shown, thecatheter arrangement 10 comprises a flexible catheter body portion 100which is adapted for insertion into a blood vessel of a patient and isformed of a non-toxic material such as polyvinyl chloride (PVC). Thecatheter body portion 100 is also preferably coated with heparin toprevent blood clot formation. At a distal tip of the catheter bodyportion 100, an inflatable balloon 102 is provided for a flow-directedmeasurement so that the catheter arrangement 10 may be inserted into theright ventricle of the heart using the customary flow-directed insertiontechnique. Within a couple of centimeters of the balloon 102 is disposeda temperature sensing device such as a thermistor or thermocouple 104for measuring the temperature of the flowing blood. This measurement isthen used in the thermodilution volumetric blood flow calculation inaccordance with known techniques, such as those described in co-pendingpatent application Ser. No. 07/510,897 to McKown et al. As shown in FIG.1, the catheter body portion 100 for insertion into the blood vesselpreferably has a length of, for example, 112 centimeters so that it islong enough to be "floated" into the right ventricle of the patient'sheart using the flow-directed insertion technique. Insertion may thus beaccomplished at bedside without the requirement of fluoroscopy.

At a proximal end of the catheter body portion 100 is provided acatheter body junction 106 through which devices such as a PA distallumen hub 108, a proximal injectate lumen hub 110, a thermistor orthermocouple connector 112, a balloon inflation valve or stopcock 114,and a heater connector 116 may be inserted into respective filament leadlumens of the catheter body portion 100. In particular, as shown in FIG.2, the catheter body portion 100 of the invention may comprise an outerlayer 202 and an intermediate layer 204 which adheres the outer layer202 to body wall portion 206 of catheter body portion 100. As shown,body wall portion 206 separates the internal area of catheter bodyportion 100 into one or more lumens for accepting the peripheral devices108-116. As will be appreciated by those skilled in the art from thefollowing description, one of the lumens permits leads from heaterconnector 116 to communicate with a downstream heating filament disposedwithin or about the catheter body portion 100. Although multiple lumensare shown, there is no reason that different leads cannot share a commonlumen.

In accordance with the invention, the heater connector 116 communicateswith a cardiac output computer so as to receive power signals forcontrolling the heating filament. Connector 112 forwards temperaturechanges measured by the thermistor or thermocouple 104 back to thecardiac output computer for calculation of the cardiac output inaccordance with a known thermodilution technique. A presently preferredthermodilution technique is that described in co-pending patentapplication Ser. No. 07/510,897, to McKown et al. and assigned to thepresent Assignee. That patent application discloses a cardiac outputcomputer which utilizes an improved stochastic technique from thatdisclosed by Yelderman in U.S. Pat. No. 4,507,974, for applying heat tothe blood stream and evaluating the results in accordance with across-correlation of the input with the measured output. The disclosureof that application is hereby incorporated by reference as if set forthentirely herein.

The heater connector 116 is shown in more detail in FIG. 3. As shown,heater connector 116 comprises electrical connector 302 within a plugportion 304 for electrically communicating with the cardiac outputcomputer. The electrical connector 302 communicates through electricalconnections in casing 306 with heater wire leads 308. Heater wire leads308 transverse the length of the support casing 310 and the supportingsheath or heater wire lumen 312 so as to electrically communicate withthe heater filament as will be described below. The supporting sheath312 is preferably made of teflon so as to be flexible yet strong. Inaccordance with the invention, the supporting sheath 312 supporting theheater wire leads 308 is inserted into a lumen of the catheter bodyportion 100 to facilitate electrical connection to the heating element.Electrical leads may be similarly "fished" through a lumen to connect tothermistor or thermocouple 104. A more complex connector will bedescribed below with respect to FIG. 8.

FIG. 4(a) illustrates the manner in which the heating filament 400 iswrapped about the outer layer 202 of the catheter body portion 100 inaccordance with a first embodiment. As shown, the heating filament 400is formed so as to be very thin and flat so that it can be wrapped in anon-overlapping manner about the outer layer 202. As shown, an injectateor pacing port 402 may also be provided proximal to heating filament400. The heating filament 400 is preferably wrapped to extendapproximately 5 to 10 centimeters along the outer layer 202 and isdisposed so as to be approximately 14 centimeters from the distal tiphaving balloon 102 of the catheter body portion 100. The heatingfilament 400 is then surrounded by a thin outer sheath 404 to preventthe heating filament 400 from directly contacting the patient's blood.

Generally, the heating filament 400 is printed on a substrate as asandwich. The substrate of the heating filament consists of a thinmaterial that is capable of being incorporated into a filament materialwhich is preferably flexible and has the ability to bond with anadhesive. It must also have good heat transfer properties which allowfor the conduction of the filament generated heat to the exterior of theouter sheath 404 so as to be applied to the blood. An additional layerof material with high thermal conductivity (e.g., metal foil) may beadded to the heater sandwich to help create a more uniform surfacetemperature. The filament materials of the invention include, but arenot limited to, Mylar and Kapton. On the other hand, the filamentmaterial, which is adhered to the substrate, can be any material whichhas a high temperature coefficient of resistance, i.e. greater than0.001 Ω/Ω-° C., and low thermal capacitance and high thermalconductivity. The material must be capable of being incorporated intothe filament substrate and must be capable of being fabricated in thinlayers so as to form a sandwich (e.g. Kapton - adhesive - filamentmetal - adhesive -Kapton). Alloys for the filament material include, butare not limited to, an alloy of 70% nickel and 30% iron or an alloy of29% nickel, 17% cobalt and 54% iron.

An adhesive material must be selected which is capable of binding toboth the outer sheath 404 and the catheter body portion 100, and to thefilament substrate, or in some applications, directly to the filamentmaterial. The adhesive must be capable of being applied in a thin, evenlayer, must be non-toxic, must not weaken with time, must tolerate heatfrom the filament, must tolerate continual flexing, and must bind wellin a wet environment (i.e., blood). Such adhesives include, but are notlimited to, pressure sensitive adhesives such as Densil.

In another embodiment, the adhesive, the outer sheath material and theelectrical resistive components may all be incorporated into onematerial. The electrical leads are then connected to the material, whichis formed as a sheath or wrapping material and applied directly to theouter layer 202 of the catheter body portion 100 or incorporated duringthe manufacturing process directly into the outer layer 202 of thecatheter body portion 100.

In accordance with the invention, the thin heating filament materials ofthe invention may be spirally wound around the catheter body portion 100to form a heating filament 400 as just described. Although the filamentsubstrate or filament heater material may be exposed directly to theblood environment as in the prior art devices, in accordance with theinvention the filament substrate and/or filament material are preferablyenclosed, surrounded by, or incorporated within an outer sheath 404 forassuring that fragments of filament or filament substrate do not becomedislodged into the blood environment. Moreover, by providing a coveringmaterial or outer sheath 404, the exterior of the catheter may be madesmoother and hence more comfortable for the patient during insertioninto the blood vessel. Of course, this structure is made possiblebecause the above-mentioned heater filament material may be formed intoa very thin filament which may be non-overlappingly wound about thecatheter body portion 100. However, the sheath 404 must also be verythin and flexible and is preferably an adhesive applied by any of anumber of techniques over the filament or filament substrate. Suchadhesives include, but are not limited to, Master Bond EP37. Theresulting catheters are then preferably coated with heparin to preventblood clot formation.

Regardless of the type of filament material used or the number of layersof materials or sandwich composition, the catheter body may be reducedin diameter in the region where the filament sandwich is wound, as shownin FIG. 4(b). The reduction in catheter body diameter is made such that,when the filament sandwich is added, the resulting total diameter in theregion of the heating filament is equivalent to the diameter of theadjacent catheter body portion without the filament material. Thisachieves a uniform transition to the region of the catheter filament,thereby eliminating problems associated with insertion, removal andthrombus formation in regions of irregularities.

In accordance with the invention, a particularly attractive method forapplying the sheath 404 is to use a flexible sheath material which canbe applied over the filament, filament substrate, andfilament-to-catheter body adhesive. Preferably, a material is used whichhas an appropriate modulus of elasticity and elongation. The materialmay be fabricated by a technique such as extrusion so that its restinglumen diameter is less than that of the catheter filament sub-assembly.The sheath material or "tube" may then be expanded using a "vacuumexpander" to a size larger than the catheter and attached filamentsub-assembly. The catheter and sub-assembly may then be passed into thevacuum expander containing the expanded sheath, positioned in place, andthen the vacuum released. The sheath then shrinks, reduces or collapsesaround the filament sub-assembly so as to maintain a certain tensionwith the underlying components. Preferably, the vacuum expander containsa chamber which allows for the placement of the sheath material so thatthe ends of the sheath material may be secured to form a closed chamberbetween the outer wall and ends of the sheath material and thesurrounding chamber. The chamber dimensions may be such as to allow forthe expansion of the sheath to a size which is large enough to acceptthe passage of the catheter body portion 100 and the attached filamentsub-assembly. The sheath then may be expanded by applying a vacuum tothe chamber and/or positive air pressure to the inside of the sheath.Expansion of the sheath may also be improved by applying heat to theexpansion chamber. Conversely, a blow molding technique may be used inaccordance with known techniques. A material which may be manufacturedto have such a thin wall, an appropriate modules of elasticity, and anappropriate elongation includes, but is not limited to, Tecoflex™.

Another method of sheath application in accordance with the inventionutilizes shrink material. The sheath may thus be fabricated to beslightly larger than the catheter body portion 100 and the attachedfilament sub-assembly. It is then applied without the vacuum expander,and when the sheath material is situated in the proper location, it isreduced in size by the application of heat. Again, the proper wallthickness and beginning dimensions are chosen such that, following thereduction in size, appropriate tension is maintained with respect to theunderneath filament sub-assembly.

Preferably, as described above, the cylindrical heating filament 400 isapproximately 10 centimeters in length and is wrapped about the outerwall 202 of the catheter body portion 100 beginning distally about 15centimeters from the distal tip of the catheter. Then, when the catheteris positioned with the distal tip in the pulmonary artery during aflow-directed measurement, a proximal fluid infusion port of thecatheter will lie in the right atrium of the heart or superior vena cavawhile the distal fluid infusion port will lie in the right ventricle.

An alternative embodiment of the invention for measuring blood flow in a"retro grade" fashion, such as in the hepatic vein, is shown in FIG. 5.As shown, the heating filament 500 and the thermistor or thermocouple502 are in reversed positions on the catheter body portion 100 becauseof the reversed blood flow direction. Since this type of catheter isinserted into the blood vessel against the blood flow, insertiongenerally requires the use of fluoroscopy for directing the catheterinto place for measurement. Since the embodiment of FIG. 5 is not aflow-directed catheter, a balloon at the distal tip is not used.

The alternative embodiment of FIG. 6 may also be used for measuringblood flow in a "retro grade" fashion, as in the left ventricle of theheart, whereby the heating filament 600 and thermistor or thermocouple602 are in reversed positions on the catheter body portion 100 as in theembodiment of FIG. 5. As in the FIG. 5 embodiment, insertion generallyrequires fluoroscopy and a balloon tip is not used. However, a pigtailtip 604 is preferably used in this embodiment to prevent vessel rupture.

During operation, since the heating filament formed as described aboveis used primarily to insert heat into the blood stream, it will rise toa temperature higher than the surrounding environment. Thus, it isnecessary to know the filament temperature since, should the temperaturebecome excessive, damage could result to the surrounding blood andtissues. Normally, a second temperature sensing device such as athermistor or thermocouple would need to be embedded next to thefilament to measure its temperature. However, by using a filamentmaterial which has a high temperature coefficient of resistance asherein described, not only can it be used as a heat supplier, but it canalso serve as its own temperature sensing device. For example,resistance of any material is measured as follows: ##EQU1## where ρ isthe resistivity,

l is the length, and

A is the cross-sectional area.

Then: ##EQU2## and if α, the mean temperature coefficient ofresistivity, is defined as: ##EQU3## where Δρ is the change in thecoefficient and ΔT is the change in temperature,

then: ##EQU4## Then, by measuring the current (i) and the voltage (v),both delivered power and resistance of the filament can besimultaneously measured as: ##EQU5##

The heating filament 400 of the invention typically consists of acylindrical design which is approximately 5-10 centimeters in length.Heater wire leads 308 are attached to the heating filament 400, and theheating filament 400 is placed at the desired distance from thethermistor or thermocouple 104 (10 cm in FIGS. 4(a) and (b)). Then, aspreviously described, the heat transfer is such that the heat passesfrom the heater filament 400 through the outer sheath 404 into theblood. Of course, the heating filament 400 must be flexible such that itdoes not increase the stiffness of the catheter body portion 100.

In accordance with another embodiment of the invention, as shown in FIG.7, the heating filament 400 may be made as a mobile module supported bya flexible supporting member 700 which can be inserted or withdrawn fromthe catheter lumen after the catheter has been inserted into thepatient. This has the advantage that the catheter can be inserted intothe patient when it is not known whether measurement of blood flow isrequired. Should the measurement of blood flow become desirable, themobile filament module can be inserted and the measurement started. Thisfeature of the invention is particularly helpful in a clinical setting,for although pulmonary artery catheters were originally designed tomeasure distal pressure, more features have been added such as bolusthermodilution cardiac output measurements, cardiac pacing and mixedvenous saturation. Thus, the clinical problem now is to know whichcatheter to use, for not all patients require all measurementmodalities.

The invention is thus designed as a pulmonary artery catheter which hasone or more ports and/or lumens which will accept the particular modules(as shown in FIG. 1) for a particular measurement modality. For example,for a 4-lumen catheter of the type shown in cross-section of FIG. 2, onelumen may be dedicated to measuring distal catheter pressure, one lumenmay be dedicated for distal balloon inflation and passage of two distalthermistor or thermocouple leads, one lumen may be dedicated to proximalfluid infusion, and the fourth lumen may be left open. Moreover, anotherlumen may receive a module for measuring mixed venous oxygen saturationincluding a fiber optic bundle. Other modules may be designed at theuser's discretion.

During use, the pulmonary artery catheter of the invention (with thevacant lumen) is inserted in the usual and customary fashion. Afterinsertion, if so desired, the physician or the user may electricallypace the heart by passing a modules or wire through the vacant catheterlumen so as to connect the proximal end of the wire to the appropriateelectronics. Such a concept of a removable pacing wire has beenpreviously described by Swendson, et al. in U.S. Pat. No. 4,759,378, forexample. On the other hand, if the measurement of mixed venoussaturation is desired, the pacing wire modules would be removed and afiber optics modules inserted in the vacant lumen for measuring mixedvenous saturation, and the proximal end of the fiber optics would beattached to the appropriate electronics. Such fiber optics techniquesfor measuring mixed venous saturation are described by Willis, et al. inU.S. Pat. No. 4,718,423, for example. However, the fiber opticstechnique taught by Willis, et al. is not removable; therefore, ifcardiac output is desired, the vacant lumen must be replaced with thethermal transducer filament or other apparatus modules for performingcardiac output measurement. Of course, the scope of the invention is notlimited to just these modalities, but includes any modalities whichcould be used at the user's discretion.

Thus, in accordance with the invention, the heating filament 400 isplaced either around the catheter body portion 100 but within an outersheath 404 or is placed within the catheter body portion 100, namely, ina lumen thereof. In either case, the heating filament 400 does notdirectly contact the patient's blood. This is in marked contrast toprevious embodiments wherein the heating elements are generally placedon the exterior of the catheter or the filaments are used as unattachedfree-floating pieces. Instead, in accordance with the present inventionthe heating filament 400 is placed such that the heat transferproperties of the catheter body portion 100, the outer sheath materialand heating filament material allow the transmission of heat to theexterior environment, namely, the blood stream. Such an arrangement hassignificant implications since an internally placed heating filamentreduces the probability of harmful blood clot formation, electricalleakage currents, or unusually high filament blood contact temperatures.

When a thermodilution catheter in accordance with the invention isconnected to a cardiac output computer via heater connector 116, anelectrical current is applied to the heating filament in the form ofpulses. When the heating filament is activated, an approximate averageof 7.5 watts of power may be delivered to the heating filament. Duringoperation, as described above, the cardiac output computer maycontinuously measure and monitor the filament temperature so as to limitthe estimated surface temperature to a maximum of 45° C. (whichcorresponds to an average surface temperature of about 41.5° C.,depending upon the material composition and thickness). For example, inthe event the heating filament core temperature exceeds a powerdependent threshold for more than, say, 15 seconds at full power, thedelivered heating filament power is reduced. If the estimated heatingfilament surface temperature exceeds 45° C. for more than, say, 4°C.-seconds at any power, the heating filament power may be shut off anda panel alarm activated. In practice, this prevents the peak surfacetemperature from exceeding 45° C. Moreover, the average catheter surfacetemperature should not exceed 41°-5° C., since the power will beswitched "ON" approximately 50% of the time. Furthermore, if the averagecardiac output exceeds 3.5 liters/minute, the catheter's average surfacetemperature will generally remain below 39.5° C. Thus, regulation ofpower to the catheter only becomes an issue when the cardiac outputbecomes less than about 3.5 liters/minute. However, since the power tothe heating filament is reduced or shut off as the estimated filamentsurface temperature reaches 45° C., the heating element of the inventioncan be made relatively fail-safe through closed-loop control of thesurface temperature.

By using a power source which is a constant voltage source, anincreasing catheter filament temperature can be directly detected as anincreasing filament resistance which reduces the power delivered to theheating filament. In this manner, the actual current and voltage to thecatheter filament may be continuously monitored. From the values ofcurrent and voltage, a delivered power may be calculated which is neededto calculate flow, and the filament resistance may be calculated andused for computing the filament core temperature. Thus, at all times,the actual filament core temperature is known. Preferably, the followingalgorithm is followed to insure that the filament temperature remainswithin safe limits:

(1) When the cardiac output computer starts, the delivered power to theheating filament is maintained at approximately 4 watts average power.

(2) The filament core temperature is monitored for several seconds.

(3) If the peak filament core temperature has not exceeded 49° C., thefilament power is increased to an average power of 7.5 watts.

(4) If at any time the peak filament core temperature exceeds 56° C.,the delivered filament power is reduced.

(5) If at any time the estimated filament surface temperature exceeds45° C. for more than, say, 4 degree-seconds, the computer shuts off anddisplays an error message.

The cardiac output may be measured continuously by turning the heatingfilament on and off in a predetermined pattern and generating acharacteristic thermodilution curve by a mathematical process such ascross-correlation as described in the afore-mentioned co-pendingapplication, U.S. Ser. No. 07/510,897 to McKown et al. A detaileddiscussion of bolus thermodilution and pulse thermodilution techniquesare described in that application.

By using an indicator dilution method in accordance with a stochasticsystem of the type described in the afore-mentioned related application,Ser. No. 07/510,897 to McKown et al. , cardiac output may be measured ina noisy environment even when a small heat input source as hereindescribed is used. The stochastic techniques of the type described inthe afore-mentioned application are different from classical empiricaltechniques in that the input signal or energy is applied over a periodof time, and the nature of the statistical properties of the input andoutput signals are of interest. Thus, during operation in accordancewith this technique, the supplied heat in accordance with the presentinvention will produce a small temperature change in the flowing bloodwhich is detected at the distal thermistor or thermocouple 104. Througha mathematical procedure known as cross-correlation, a scaledcharacteristic thermodilution "wash-out" curve is reconstructed. Thecardiac output may then be calculated by measuring the area under this"wash-out" curve if the amount of heat delivered to the blood by theheating filament is also known. An indicator thermodilution equation forcalculating flow is described in the afore-mentioned application.

In the calculation of cardiac output using such thermodilutiontechniques, it is necessary to know certain properties about themeasuring transducer, such as the thermistor or thermocouple 104, andthe heat application or heating filament efficiency, for in themanufacturing process it is difficult to produce either thermistors orthermocouples 104 or heating filaments 400 which uniformly have the sameproperties. Thus, to reduce the errors which would be introduced intothe calculation of cardiac output due to these variances, it isnecessary to calibrate or measure the physical properties of both thethermistor or thermocouple 104 and the heating filament 400. Since in aclinical environment each cardiac output computer may be attached overtime to various pulmonary artery catheters and to eliminate the need forthe user to manually transcribe these calibration numbers to thecomputer, a coding technique has been developed in accordance with theinvention to pass the calibration information.

Prior art thermodilution catheters and pulse oximeter sensors have usedresistors to code the values for thermistors or LEDs. For example, Newet al. in U.S. Pat. No. 4,700,708 use a resistor to calibrate LEDwavelengths on a pulse oximeter. However, the present inventors know ofno previous attempt to code the filament calibration for transferringthe calibration information of the heating filament solely or thecalibration information of the heating filament and thermistor orthermocouple together. Thus, in accordance with the present invention,calibration of the heating element may be conducted by measuring theheater resistance at a known temperature. The catheter assembly can thenuse the previously calibrated thermistor or thermocouple and a built-inohm meter to establish a calibrated reference point for the heaterelement. This approach has the advantage of calibrating the heaterimmediately prior to use in a patient at the patient's body temperature.Such an accurate calibration of heater resistance and temperature isnecessary to accurately monitor heater temperature to insure patientsafety.

The calibration circuit may include passive electronic components suchas resistors, inductors and capacitors such that the value of thecomponents correspond to a particular calibration value or numberaccording to a predetermined table. On the other hand, active electroniccomponents including numerous nonlinear components may be used such thata particular performance corresponds to a particular calibration numberor value. Such calibration information is preferably stored in a memorycomponent such as a ROM (Read Only Memory), a RAM (Random AccessMemory), a nonvolatile memory device, or another type of memory ordigital device. The calibration information preferably includes codesthat represent the filament resistance, filament efficiency, and otherparameters. If properly selected, one or more electronic components maybe used to encode the calibration information of the thermistor orthermocouple, such as its β value, and the filament resistance, filamentefficiency and other parameters.

Thus, the calibration information for both the thermistor orthermocouple 104 and the heating filament 400 may be encoded by one ormore active or passive electronic components or these values may bestored in a suitable memory device. The cardiac output computer may thendecode this information and incorporate it into the calculation ofcardiac output. However, this step may be eliminated if the actualappropriate software is contained in the catheter itself. For example, amemory device such as a ROM may be contained in the catheter with aportion of the software utilized by the cardiac output computer residentwithin it. Such information might include program segments or historicalpatient data. Thus, when the catheter is connected to the cardiac outputcomputer, prior to the beginning of processing for determining thecardiac output, the software or program segment contained in thecatheter memory device (ROM) may be transferred to the main softwareprogram of the cardiac output computer. This feature of the inventionalso provides an additional safety feature, for the cardiac outputcomputer will not start until it has transferred the program segment andincorporated this segment into its own program.

The calibration circuitry of the type just described can be seen by wayof example in FIG. 8. As should be apparent to one of ordinary skill inthe art, the calibration circuit of FIG. 8 is quite different from thatused in typical prior art thermodilution catheters. In particular,classic thermodilution catheters use calibration resistances which areconnected in series with the thermistor or thermocouple. In suchdevices, the reference resistor is calibrated to match the thermistor orthermocouple for a standard temperature. In this manner, compensationfor variability in the thermistors or thermocouples may be achieved.However, by using the calibration circuit of the invention wherein a ROMcontaining calibration data is included within the connector of thecatheter, such a reference resistor for calibration purposes is notneeded. Such a ROM is shown as ROM 802 of connector 116 in FIG. 8.

Preferably, the software module referred to above is stored in the ROM802 and includes such things as the format version for the calibrationdata, trademark information, historical patient data (such as cardiacoutput for the previous several hours) or whatever information isdesired for controlling the cardiac output program. Thus, by placing theencoded calibration data within the ROM 802 and placing the ROM 802 onthe catheter, the thermistor or thermocouple reference resistance may beeliminated. In addition, only a catheter having a ROM 802 storing thenecessary information for operating the program of the cardiac outputcomputer may be used in conjunction with the cardiac output computer toobtain the desired calculation.

Although a number of exemplary embodiments of the invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many additional modifications are possible in theexemplary embodiments without materially departing from the novelteachings and advantages of this invention. For example, rather thanwrapping the heating element 400 around the catheter body portion 100,the heating element may be included in the body wall portion 202 of thecatheter body portion 100. In addition, the heating element 400 may bemade in multiple contiguous sections, whereby by measuring thetemperature of each section it is possible to determine whether onesection is malfunctioning. Such malfunctions could be due to filamentabnormalities or due to physiologic aberrations such as clotting. Thediscrepancy in temperature would alert the user to a potential problem.However, such a section arrangement would require additional electricalleads, and the catheter would need to be modified accordingly.Alternatively, the heating filament of the invention may be used inconjunction with a guide wire for angioplasty, where the thermistor orthermocouple will be miniaturized and placed on the guide wire, and theheater placed upstream on the guide wire. The resulting device may thenbe inserted into a catheter lumen of the type described herein. Inaddition, the heating filament may be placed ahead of or behind balloon102 as desired.

Accordingly, all such modifications are intended to be included withinthe scope of this invention as defined in the following claims.

Description of the Embodiment of FIGS. 9-17

Referring to FIG. 15, it will be seen that the cardiac output computer("COC") 710 is connected to catheter arrangement 10 such that it is incommunication with the heating element 400, the thermistor orthermocouple 104 and the ROM 802. The ROM 802 and the technical detailsassociated therewith are fully disclosed in U.S. patent application Ser.No. 07/769,536 to inventors Yelderman and Quinn entitled "A DiagnosticCatheter with Memory." The disclosure in that application is herebyincorporated into this document as if set forth fully herein. Referringagain to FIG. 15, it will also be seen that COC 710 includes a systemwhich is constructed and arranged to maintain the surface temperature ofheating element 400 at a level which is physiologically safe. Morespecifically, COC 710 is constructed and enabled to perform a safetyshut-off method which provides for the automatic shut-off of power tothe heater element 400 under conditions of very low measured flow, orwhenever the unit is inadvertently operated with the catheter in air.COC 710 is also constructed and enabled to prevent the peak surfacetemperature of heating element 400 from exceeding a maximum safetemperature (in the preferred embodiment 45° Centigrade) duringconditions of low flow, which in the preferred embodiment is 0.5 to 2.5lpm. COC 710 is further constructed and enabled to perform a heaterelement resistance verification test which provides an in vivo systemreadiness test that checks the calibration of both the COC and thecatheter to ensure proper operation of the safety shut-off system.

1. The Core Temperature Monitor

As will be seen in the description provided below, both the safetyshut-off system and the two state power control system require themonitoring of a core temperature within heating element 400. Accordingto one advantageous feature of the invention, the core temperaturemonitoring is conducted without a separate temperature measuring device.As will be discussed below, this is done by continuously measuring thevoltage and current supplied to heating element 400 to calculate boththe delivered power and the resistance. The core temperature is thencalculated by the COC 710 based on the resistance of heating element400, the temperature coefficient of electrical resistance ("TCR") of themetal used in heating element 400, and the known resistance of theheating element 400 at a reference temperature. In the preferredembodiment, the reference temperature and reference resistance areprecalibrated into ROM 802 during the manufacturing process.

Referring briefly to FIG. 15, it will be seen that the COC 710 includesa voltage and current measuring unit 712 having a voltage measuringsubunit 714 and a current measuring subunit 716. COC 710 furtherincludes a resistance and power monitoring unit 718 which includes aresistance monitoring subunit 720 and a power monitoring subunit 722.Resistance monitoring subunit 720 receives a voltage measurement fromvoltage measuring subunit 714 and a current measurement from currentmeasuring subunit 716. Similarly, power monitoring subunit 722 receivesa voltage measurement from voltage measuring subunit 714 and a currentmeasurement from current measuring subunit 716. The resistance of theheating element 400 is calculated in resistance monitoring subunit 720by dividing voltage by current, according to Ohm's Law. This resistancevalue is supplied to a core temperature monitoring unit 724, also in COC710. Core temperature monitoring unit 724 includes core temperaturecalculating means 726 which calculates the core temperature of heatingelement 400 according to the formula set forth below in Equation 1.

    T.sub.core =T.sub.0 +(R-R.sub.0)/(R.sub.0 *TCR);           (Equation 1)

The specific numeric values for R₀, T₀ and TCR are unique to eachthermodilution catheter, and they are established and stored during themanufacturing process in the catheter ROM 802.

Referring again to FIG. 15, COC 710 further includes a surfacetemperature calculating unit 728 for calculating the surface temperatureof heating element 400. As may be seen in FIG. 15, surface temperaturecalculating unit 728 receives input from power monitor subunit 722,which calculates the power supplied to heating element 400 bymultiplying voltage and current. Surface temperature calculating unit728 further receives the core temperature of heating element 400 fromthe core temperature monitoring unit 724. Surface temperaturecalculating unit 728 calculates the surface temperature of heatingelement 400 according to the following formula:

    T.sub.surface =m.sub.core * T.sub.core +m.sub.power * P.sub.element +b;(Equation 2)

Where T_(core) is the core temperature estimate obtained from coretemperature monitoring unit 724; P_(element) is the power applied toheater element 400, obtained from power monitoring subunit 722; m_(core)is a constant relating core temperature to surface temperature underanticipated clinical conditions; m_(power) is a constant relating thepower delivered to the heater element to the incremental increase insurface temperature that will be created by the power under anticipatedclinical conditions; and b is a numeral constant.

The specific numeric values for m_(core), m_(power) and b are unique toeach thermodilution catheter model, and they are established and storedduring the manufacturing process in ROM 802. The preferred methodologyfor establishing such values for a particular thermodilution cathetersystem will be described below with reference to example I.

EXAMPLE I

By carefully mounting a three millimeter (0.003 inch) diameterthermocouple (Omega Engineering, type T) to the surface of the heaterelement 400, simultaneous core and surface temperatures can be measuredat different flows and heater powers. These data allow the developmentof an empirical model for the surface temperature given the electricallymeasured heater power and core temperature.

FIGS. 11 and 12 show the surface temperature data that accompany thecore temperature data of FIGS. 9 and 10, the turbulent and laminar flowcases, respectively. To average the effects of convection currents atzero flow, the thermocouple was positioned on the side of the horizontalcatheter. These surface temperature data were obtained by visuallyaveraging a one sample per second digital temperature display over a 15second period.

These data were acquired using a dc power supply to continuously providepower to the heater element. A system according to the preferredembodiment of the invention, on the other hand, powers the heater with asignal derived from a pseudo random binary sequence ("PRBS") of periodlength 15. Since the PRBS length 15 signal is activated only 8/15 of thetime, an average surface temperature can be defined as:

    T.sub.surface/ave =(8/15) * T.sub.surface/on +(7/15) * T.sub.bath ;

Where T_(surface/on) is the above surface temperature data and T_(bath)is the temperature of the fluid bathing the catheter.

Given that the laminar and turbulent flow cases represent extremes andthat in vivo blood flow is highly pulsatile, an average of the two casesis considered an appropriate model of the blood flow around the PA3Hcatheter in clinical use. FIG. 13 shows the surface temperature versusthe corresponding core temperature when the laminar and turbulent datasets are averaged as follows:

    T.sub.surface = C.sub.surface (laminar)+T.sub.surface (turbulent)!/2;

and

    T.sub.core = T.sub.core (laminar)+T.sub.core (turbulent)!/2

These data allow the development of a clinical model for the surfacetemperature in terms of the measured core temperature and the appliedpower.

FIG. 13 shows the regression lines for the 10 and 15 watt data sets.Note that the slopes of the two lines are nearly equal, i.e., atemperature versus power slope of approximately 0.85. By assuming thedependence of the regression intercept is linear with respect to power,we can define a linear model for the surface temperature, T_(surface) :T_(surface) =n_(core) * T_(core) +m_(power) * P_(element) +b, which is,of course, Equation 1. Referring again to FIG. 13, it is apparent thatm_(core) is, for this data set, approximately 0.85. Analysis of theregression intercepts (of the lines on FIG. 13 versus power) determinesm_(power) as approximately -0.25 (C/watts) and b as approximately 1.1(C). Equation 1 is thus used by the COC 710, and, specifically, surfacetemperature calculating unit 728, to estimate the surface temperature ofheater elements 400 on a sample by sample basis.

FIGS. 9 and 10 illustrate the heater element core temperature as afunction of flow for continuously applied or "constant on" power of 10and 15 watts. The data in FIG. 9 was taken with the heater elementpositioned in a turbulent flow region of the test chamber, whereas theelement was positioned in a laminar flow region for the data in FIG. 10.

It should be noted that the turbulent and laminar flow cases haveequivalent core temperatures are zero flow (approximately 54° and 64°centigrade for 10 and 15 watts), but that the turbulent flow cools theheater element better than the laminar flow and the flow ranges ofclinical interest. In general, the core temperature is 5°-8° centigradewarmer at 15 watts power input than at 10 watts, with the largestdifference being at zero flow.

The data in FIGS. 9 and 10 were obtained using a standard INTERFLO brandPA3H catheter (heater part number 40245-4001) mounted in a 250 ml testchamber of a temperature controlled flow (37° C.) flow bench. The powerto the heater element was from a dc power supply with current andvoltage measurements being obtained with standard electronic testequipment. It should also be noted that the "constant on" temperaturedata represents the peak, not average, core temperatures that wouldoccur with a system according to the preferred embodiment of theinvention, since the preferred embodiment pulses the heater over 8/15 ofits duty cycle. The method by which the core temperature of heatingelement 400 is calculated will be demonstrated with reference to thefollowing example:

EXAMPLE II

A heating element 400 was provided which consisted of a metal foilenclosed by a 0.001 inch layer of Kapton™. It was wrapped around arecessed portion of the catheter, bonded with adhesives and covered witha 0.001 inch thickness of PEVA heat shrink sheath. The metal wasselected as a 70% nickel/30% iron alloy in order to provide a hightemperature coefficient of electrical resistance (in this specificexample, TCR=4200 parts per million per degree centigrade=0.00420ohm/ohm/centigrade, nominal).

This arrangement allows the temperature of the heater element core,i.e., the metal itself, to be monitored by measuring the electricalresistance of the heater element. The cardiac output computer 710computes the core temperature, T_(core), as:

    T.sub.core =T.sub.0 +(R-R.sub.0)/(R.sub.0 * TCR) ; (Equation 1)

Where R is the (time varying) resistance of the heater element 400 andR₀ is the reference resistance of the heater element at the referencetemperature T₀, which in this example is a body temperature of 37° C.centigrade.

Using this technique, it is estimated that the core temperature T_(core)can be measured to an accuracy of +/-1.3° centigrade. This assumes anerror margin for R₀ of +/-0.1 ohms; an error for the TCR of +/-0.0001(1/C); and an error in the reference temperature T₀ of +/-0.1° C.

The safety shut-off system within COC 710 further includes a detectionunit 730, in communication with the surface temperature calculating unit728, for determining whether a potential physiologically harmfultemperature condition exists at heating element 400. Detection unit 730is constructed and arranged to instruct a power control unit 740 tocease supplying power to heating element 400 when the calculated surfacetemperature of heating element 400 exceeds a temperature threshold,defined as:

    T.sub.safe threshold =T.sub.max safe,

if T_(b) is greater or equal to 37° C., or

    T.sub.safe threshold =T.sub.max safe +(T.sub.b -37),

otherwise; where:

T_(b) is the sample pulmonary artery blood temperature, and T_(max) safeis a control parameter.

Referring again to FIG. 15, it will be seen that the detection unit 730includes a limit comparison subunit 732 having a conditional integratorincorporated therein, and a subunit 734 for comparing a time/temperatureproduct from the conditional integrator to a predetermined maximumvalue. Detection unit 730 will instruct power control unit 740 to ceasesupplying power to heating element 400 only if T_(surface) exceedsT_(safe) threshold for more than a specified integrated time temperatureproduct. The conditional integrator within limit comparison subunit 732resets to zero when any sample by sample estimate of T_(surface) isbelow T_(safe) threshold, Thus, shut-off requires continuous T_(surface)samples above the T_(safe) threshold such that their integrated areaexceeds a hot area threshold, which is stored in ROM 802. When thisoccurs, detection unit 730 instructs power control unit 740 to shut-offpower to the heating element 400, and the COC 710 exits the operativemeasurement mode. The time it takes for the integrated T_(surface) totrigger the hot area threshold is a measure of the rate of temperaturechange in the heater element. If this time is less than a time hotthreshold parameter, than the COC 710 considers the catheter to beoperating in air, and the message "CHECK HEATER POSITION" appears on anoperator warning display unit 736. If the time exceeds the time hotthreshold, the catheter is considered to be in the patient, and themessage "LOW FLOW DETECTED" appears instead on the operator warningdisplay unit 736.

The values of the safety shut-off parameters used in the current COC 710are, preferably:

    T.sub.max safe =45° C.

Hot area threshold=4 C-seconds

Time hot threshold=0.7 seconds.

T_(max) safe, the hot area threshold and time hot threshold are alsostored in the ROM 802.

2. The Two-State Power Control Method

The temperature dependence of the heater element 400 to blood flowvelocity motivates the design of a two state power control method in theCOC 710. Based on the flow data exhibited in FIGS. 9 and 10 and the factthat clinical flows are normally above 2.5 lpm, the inventors haveendeavored to achieve the following desirable characteristics in thepower control method:

1. Initial operation at a power level selected by the initial powerselection method described below;

2. When operating at a higher power level, if the flow drops below about2.5 lpm (T_(core) greater than 56° C. at 15 watts), switch to a lowerpower level;

3. If, after switching to the lower power level, the flow increasesabove 3.5 lpm (T_(core) less than 49° C. at 10 watts), switch back tothe higher power level.

In the preferred embodiment, the low power level is 10 Watts, and thehigh power level is 15 Watts. A fourth requirement, which is imposed bythe signal processing system according to the preferred embodiment ofthe invention, required to estimate flow is that a power control unit738 (shown in FIG. 15) only adjusts power to the heater at PRBS "run"boundaries.

The power control method in the COC 710 compares the measured coretemperature to two power dependent core temperature thresholds: T_(core)high and T_(core) low, where "high" and "low" refer to flow, not power.FIG. 14 is a graphical depiction of core temperature versus power forthe two thresholds T_(core) high and T_(core) low. As may be seen in thelinear depiction of T_(core) low and T_(core) high in FIG. 14, thosethresholds may be calculated as follows:

    T.sub.core high =m.sub.high * P.sub.element +b.sub.high ;

and

    T.sub.core low =m.sub.low * P.sub.element +b.sub.low.

Using the data from example 1, upon which the graphical depiction inFIG. 14 is based, m_(high) will have a value of approximately 1.29;b_(high) =36.57; m_(low) =1.41; and b_(low) =36.8.

This comparison is performed on a sample by sample basis during the PRBSrun. If either the high or low threshold is exceeded, a correspondingflag is set. If the current power level is 15 watts and the coretemperature during the previous run exceeded the T_(core) low threshold,the power control unit 738 instructs the power control unit 740 toreduce the power to 10 watts. If the current power level is 10 watts andthe core temperature during the previous run did not exceed the T_(core)high threshold, the power control unit 738 instructs the power controlunit 740 to increase the power back to 15 watts. It should be understoodthat the invention is not limited to a power control unit which providespower at the disclosed levels and that different power level valuescould be used, that more than two discrete power levels could be used,and that power could be continuously varied within the scope of theinvention.

It should be noted that power control unit 738 thus provides ahysteresis such that slight variations in T_(core) around eitherthreshold do not cause alternate run power switching.

It should be noted, that by taking out the power dependence (i.e.,making the thresholds a linear function of power) the switching is flowdependent, but independent of the settings for the high and low powerstates (which are presently 10 and 15 watts). The "*"/"zero" data pointsin FIG. 14 are the average core temperatures that are obtained at a flowof 1.5/2.5 lpm for approximately 10 and 15 watts. The straight line fitsare the T_(core) high and T_(core) low thresholds versus power. The useof 1.5/2.5 lpm data instead of 2.5/3.5 lpm data is to allow for theeffects of core temperature noise.

3. The Heater Resistance Verification System

According to another advantageous feature of the invention, the COC 710is constructed with an HRV unit 742 so as to be able to determinewhether each combination of catheter and instrument passes a safetyshut-off verification test. It is actually a system readiness test whichis performed upon the initial start of system operation, either afterpower is on or when the COC 710 recognizes a new catheter.

FIGS. 16 and 17 show data from a feasibility study that illustrates thebasic concept of the heater resistance verification system. That conceptinvolves the linear relationship between measured heater resistance andthe power applied to the heater element under conditions of constantflow.

Given the fact that flow is constant over the period of measurement, thezero power intercept of the resistance versus power regression lineprovides a measure of the heater element resistance at the current bloodtemperature.

The data in FIG. 16 was obtained when the hydromodel catheter wasenergized with a dc power supply and precision test instruments wereused to measure the heater elements current and voltage. The data inFIG. 17 was also obtained with the hydro model catheter, using the COC710. In both cases, resistance measurements were obtained at powersettings of approximately 5, 10, and 15 watts for various constant flows(0, 0.3, 1, 2, 6, and 9 lpm). The measurements were obtained with acontinuously applied power allowing 5 seconds or more for thetemperature to stabilize after a flow transition.

A straight line was fitted to the data at each flow using the leastsquares error technique, and the resultant slope and intercept data aredisplayed in the figures. Since the temperature of the bath in the hydromodel chamber is controlled to 37 +/-0.05° centigrade, the zero powerresistance measurement should be equal to the catheter R₀ (T₀ =37°C.)=37.63 ohms. The results in FIGS. 16 and 17 show the agreement within+/-0.05 ohms, except for the zero flow DC supply measurement, which iswithin +0.12 ohms. The zero flow bath temperature is the least stable.

The heater resistance verification (or "HRV") algorithm conducted by HRVunit 742 provides an estimate of Ro from the zero power intercept of theresistance versus power data obtained when the catheter is at thepatient's blood temperature. Thus, equation 1 is solved for an estimateof Ro:

    Ro.sub.b =R.sub.b / (Tb-To) * TCR+1!;

where

Ro_(b) is the estimate of the reference resistance Ro;

Rb is the measured zero power resistance intercept;

Tb is the measured blood temperature;

To is the reference bath temperature (stored in ROM 802); and

TCR is the temperature coefficient of resistance (also stored in ROM802).

The estimate of Ro_(b) is then compared to the catheter referenceresistance R_(o), stored in the ROM 802, and a pass-fail decision ismade on whether the calibration of this particular/catheter instrumentcombination is sufficiently accurate to support the safety shut-offalgorithm. A fail decision results in the CCO mode being inactivated andin a message to the operator which advises the use of the injectatemode.

Note that if Ro_(b) is sufficiently lower than Ro, the safety shut-offalgorithm fails to shut-off the instrument under conditions of zeroflow, such as when the patient is on bypass. On the other hand, Ro_(b)sufficiently higher than Ro results in the safety shut-off algorithmfalse triggering at normal levels of cardiac output and in theinterruption of measurement mode.

In detail, the HRV algorithm consists of the following steps:

1. Record the blood temperature, Tb₁. The blood temperature is measuredby the catheter thermistor or thermocouple 104.

2. Activate the heater element to a requested power, (which in thepreferred embodiment is 5 watts), for a time period (which in thepreferred embodiment, is 4 seconds) and record the measured heaterelement resistance and delivered power averaged over the last 2 seconds.

3. Repeat step 2 at a second power level (which in the preferredembodiment is 7.5 watts).

4. Repeat step 2 at a third power level (which in the preferredembodiment is 10 watts).

5. Record the blood temperature again.

6. Validate the data by testing that the absolute value of thedifference between the two recorded blood temperatures is less than amaximum threshold value which in the preferred embodiment is 0.2°centigrade).

7. Use the linear least squares algorithm to compute estimates of (a)the zero power resistance intercept Rb; (b) the uncertainty in theestimate of Rb, U_(rb) ; (c) the slope of the resistance versus powerline, m_(RP) ; and (d) the uncertainty in the estimate of m_(RP),U_(mrp).

8. Validate the estimates by testing that (a) U_(rb) is less than orequal to U_(rb) threshold ; (b) m_(RP) min is less than or equal toM_(RP) is less than or equal to m_(RP) max ; and (c) U_(RP) is less thanor equal to U_(mRP) threshold, where, in the preferred embodiment,U_(Rb) threshold =0.3 ohms; m_(RP) min =-0.05; M_(RP) max =0.5; andU_(mRP) threshold =0.15.

9. If the data and/or estimates do not pass the validation tests (steps6-8)) return to step 1. If, after three tries they still fail, go tostep 12. If they pass validation continue.

10. Using T_(b) =0.5* (Tb₁ +Tb₂) in the To and TCR from the catheter ROM802, compute Ro_(b) using Equation 3.

11. Compute Δ_(Ro) =Ro-Ro_(b) and test that: Error_(Ro) neg is less thanor equal to Δ_(Ro) if less than or equal to error_(Ro) pos, whereerror_(Ro) neg equals -1 ohm and error_(Ro) pos equals +1 ohm.

12. If step 11 fails provide the following fault message to the user:"FAULT: CATHETER VERIFICATION ERROR--USE INJECTATE MODE." If step 11passes, proceed to normal flow measurement operation.

Note that the safety shut-off processing is in effect during the dataacquisition phase (steps 2-4) of the HRV processing. Safety shut-offshould always have higher priority than HRV.

The initial power selection algorithm performed by the COC 710 will nowbe described. The regression line obtained from the above HRV algorithmallows the COC 710 to intelligently select the high or low power states(e.g., 15 to 10 watts) for the initial CCO operation. This is doneaccording to the following algorithm:

1. The heater resistance vs power regression line is used to estimatethe heater resistance R₁₅, at 15 watts, i.e.,

    R.sub.15 =m.sub.RP * 15+R.sub.b;

2. equation (1) is used to estimate the associated core temperatureT_(core)(15), at 15 watts, i.e.,

    T.sub.core(15) =To+(R.sub.15 -Ro)/(Ro * TCR)

3. equation (2) is used to estimate the associated surface temperature,T_(surface)(15), at 15 watts, i.e.,

    T.sub.surface(15) =m.sub.core * T.sub.core(15) +m.sub.power(15) +b;

4. T_(surface)(15) is compared to the T_(max) safe parameter of thesafety shutoff algorithm and the initial power is selected according to:

    T.sub.surface(15) =T.sub.max safe →select 10 watts (low state)

    else→select 15 watts (high state).

This procedure for selecting the initial power setting should eliminatethe CCO safety shutoff that would otherwise result from initiallyoperating at 15 watts on a patient having a low cardiac output.

It is to be understood, however, that, even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly and that changes may be made in detail, especially in matters ofshape, size and arrangement of parts within the principles of theinvention to the full extent indicated by the broad general meaning ofthe terms in which the appended claims are expressed.

What is claimed is:
 1. A system for keeping a surface temperature of ablood contacting surface of a thermodilution catheter within safephysiological limits, comprising:a core temperature monitor formonitoring a core temperature of a core of an electric resistance-typethermodilution heater element of the thermodilution catheter; a powersource for supplying electric power to said thermodilution heaterelement; a power monitor for monitoring an amount of the electric powerthat is supplied to said thermodilution heater element; conditiondetermining means, for determining when a potentially physiologicallyharmful temperature condition exists; and control means, incommunication with said core temperature monitor, said power monitor,said power source, and said condition determining means, for controllingthe amount of the electric power that is supplied to said thermodilutionheater element by said power source, to keep the surface temperaturewithin safe physiological limits.
 2. A system according to claim 1,further comprising a resistance monitor which is adapted forcommunication with said heater element and said core temperaturemonitor, for monitoring the electrical resistance of said thermodilutionheater element.
 3. A system according to claim 2, wherein said coretemperature monitor comprises a core temperature calculating means forcalculating the core temperature of said thermodilution heater elementas a function of electrical resistance of said thermodilution heaterelement as determined by said resistance monitor.
 4. A system accordingto claim 3, wherein said core temperature calculating means functions tocalculate the core temperature based on:(a) a reference resistance ofthe core at a reference temperature of the core; and (b) a temperaturecoefficient of electrical resistance of the core.
 5. A system accordingto claim 4, wherein said core temperature calculating means furtherfunctions to calculate the core temperature according to the formula:

    T.sub.core =T.sub.0 +(R-R.sub.0)/(R.sub.0 *TCR);

where T_(core) is the core temperature; T₀ is the reference temperature;R is said thermodilution heater element resistance measured by saidresistance monitor; R₀ is the reference resistance; and TCR is thetemperature coefficient of electrical resistance of said thermodilutionheater element.
 6. A system according to claim 2, further comprisingmeans for measuring voltage and current that are supplied to saidthermodilution heater element and wherein said resistance monitorcomprises means, in communication with said means for measuring, forcalculating the electrical resistance of said thermodilution heaterelement based on the voltage and current that are measured by said meansfor measuring.
 7. A system according to claim 1, further comprisingmeans for measuring voltage and current that are supplied to saidthermodilution heater element and wherein said power monitor comprisesmeans, in communication with said means for measuring, for calculatingthe amount of the electric power that is delivered to saidthermodilution heater element based on voltage and current that aremeasured by said means for measuring.
 8. A system according to claim 1,wherein said condition determining means comprises means for determiningwhen the surface temperature exceeds a predetermined physiologicallysafe temperature limit.
 9. A system according to claim 1, furthercomprising:surface temperature calculating means, in communication withsaid core temperature monitor and said power monitor, for calculatingthe surface temperature; and wherein said condition determining means isalso in communication with said surface temperature calculating means.10. A system for keeping a surface temperature of a blood contactingsurface of a thermodilution catheter within safe physiological limits,comprising:a core temperature monitor for monitoring a core temperatureof a core of an electric resistance-type thermodilution heater elementof said thermodilution catheter; a power source for supplying electricpower to said thermodilution heater element; a power monitor formonitoring the amount of electric power that is supplied to saidthermodilution heater element; and control means, in communication withsaid core temperature monitor and with said power monitor, forcontrolling the amount of the electric power that is supplied to saidthermodilution heater element by said power source to maintain thesurface temperature within safe physiological limits.
 11. A systemaccording to claim 10, further comprising a resistance monitor, incommunication with the core temperature monitor, for monitoringelectrical resistance of said thermodilution heater element.
 12. Asystem according to claim 11, wherein said core temperature monitorcomprises core temperature calculating means, which is in communicationwith said power monitor, for calculating the core temperature of saidthermodilution heater element as a function of the electrical resistanceof said thermodilution heater element determined by said resistancemonitor.
 13. A system according to claim 12, wherein said coretemperature calculating means functions to calculate said thermodilutionheater element core temperature based on:(a) a reference resistance ofthe core at a reference temperature of the core; and (b) a temperaturecoefficient of electrical resistance of the core.
 14. A system accordingto claim 13, wherein said core temperature calculating means furtherfunctions to calculate said thermodilution heater element coretemperature according to the formula:

    T.sub.core =T.sub.0 +(R-R.sub.0)/(R.sub.0 *TCR);

where T_(core) is the core temperature; T₀ is a reference temperature; Ris the resistance of said thermodilution heater element measured by saidresistance monitor; R₀ is the reference resistance; and TCR is thetemperature coefficient of electrical resistance of said thermodilutionheater.
 15. In combination, the system according to claim 10 and thethermodilution catheter.
 16. The combination according to claim 15,wherein said thermodilution catheter further comprises a plurality oflumens.
 17. The combination according to claim 15 further comprising amodule for measuring mixed venous oxygen saturation including a fiberoptic bundle, said fiber optic bundle being at least partially in one ofsaid plurality of lumens.
 18. In combination, the system according toclaim 10 and the thermodilution catheter having the core element. 19.The combination according to claim 18 wherein said thermodilutioncatheter comprises a plurality of lumens and said core is inside of oneof said plurality of lumens.
 20. The combination according to claim 18wherein said core is wound around an outside of said plurality oflumens.
 21. A method for keeping a surface temperature of a bloodcontacting surface of a thermodilution catheter within safephysiological limits, comprising the steps of:(a) monitoring a coretemperature of a core of an electric resistance-type thermodilutionheater element of said thermodilution catheter; (b) supplying electricpower to said thermodilution heater element; (c) monitoring the amountof electric power that is supplied to said thermodilution heaterelement; (d) determining whether a potentially physiologically harmfultemperature condition exists; and (e) controlling the amount of theelectric power that is supplied to said thermodilution heater element tokeep the surface temperature within safe physiological limits.
 22. Amethod according to claim 21, wherein step (a) comprises the stepsof:(a)(i) monitoring the electrical resistance of said thermodilutionheater element; and (a)(ii) calculating the core temperature accordingto the formula:

    T.sub.core =T.sub.0 +(R-R.sub.0)/(R.sub.0 *TCR);

where T_(core) is the core temperature; T₀ is a reference temperature; Ris the electrical resistance; R₀ is the electrical resistance at thereference temperature; and TCR is temperature coefficient of theelectrical resistance.
 23. A method according to claim 21, wherein step(c) comprises the steps of:(c)(i) measuring voltage and current that aresupplied to said thermodilution heater element and (c)(ii) calculatingthe power supplied to said thermodilution heater element based on themeasured values of the voltage and the current.
 24. A method accordingto claim 21, wherein step (d) comprises determining whether the surfacetemperature exceeds a predetermined safe physiological surfacetemperature limit.
 25. A method according to claim 21, furthercomprising the steps of:(f) calculating the surface temperature, basedat least in part on the core temperature and the amount of the electricpower that is supplied to the thermodilution heater element; and whereinthe step of determining is based at least in part on the surfacetemperature.
 26. A method according to claim 25, wherein step (f) isperformed by calculating the surface temperature according to theformula:

    T.sub.surface =m.sub.core * T.sub.core +m.sub.power * P.sub.element +b;

where T_(surface) is the surface temperature, m_(core) is a constantrelating the core temperature to the surface temperature underanticipated clinical conditions; T_(core) is the core temperature;m_(power) is a constant relating the amount of electric power that issupplied to said thermodilution heater element to incremental increasesin the surface temperature that are created by the amount of electricalpower under anticipated clinical conditions; P_(element) is the amountof electrical power that is supplied to said thermodilution heaterelement; and b is a numerical constant.
 27. A system for controlling asurface temperature of a blood contacting surface of a thermodilutioncatheter comprising:a core temperature monitor for monitoring a coretemperature of a core of an electric resistance-type thermodilutionheater element of said thermodilution catheter; a power source forsupplying electric power to said thermodilution heater element; a powermonitor for monitoring an amount of the electric power that is suppliedto said thermodilution heater element; control means, in communicationwith said core temperature monitor, said power monitor and said powersource, for controlling the amount of the electric power that issupplied to said thermodilution heater element by said power source, tokeep the surface temperature within safe physiological limits; andsurface temperature calculating means, in communication with said coretemperature monitor and said power monitor, for calculating the surfacetemperature.
 28. A method for keeping a surface temperature of a bloodcontacting surface of a thermodilution catheter within safephysiological limits, comprising:(a) monitoring a core temperature of acore of an electric resistance-type thermodilution heater element ofsaid thermodilution catheter; (b) supplying electric power to saidthermodilution heater element; (c) monitoring an amount of the electricpower that is supplied to said thermodilution heater element by saidpower source; and (d) controlling the amount of the electric power thatis supplied to said thermodilution heater element by said power sourcebased on the core temperature so that the surface temperature is keptwithin safe physiological limits.